linux-kernel - Re: [PATCH tip/core/rcu 0/4] Programmatic nestable expedited grace periods

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Message-ID: <20150220171442.GM5745@linux.vnet.ibm.com>
Date:	Fri, 20 Feb 2015 09:14:42 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com,
	arjan@...ux.intel.com
Subject: Re: [PATCH tip/core/rcu 0/4] Programmatic nestable expedited grace
 periods

On Fri, Feb 20, 2015 at 05:54:09PM +0100, Peter Zijlstra wrote:
> On Fri, Feb 20, 2015 at 08:37:37AM -0800, Paul E. McKenney wrote:
> > On Fri, Feb 20, 2015 at 10:11:07AM +0100, Peter Zijlstra wrote:
> > > So I though we wanted to get rid / limit the expedited stuff because its
> > > IPI happy, and here its spreading.
> > 
> > Well, at least it no longer IPIs idle CPUs.  ;-)
> > 
> > And this is during boot, when a few extra IPIs should not be a big deal.
> 
> Well the one application now is during boot; but you expose the
> interface for all to use, and therefore someone will.

I could make rcu_expedite_gp() and rcu_unexpedite_gp() be static,
I suppose.  Except that I need to test them with rcutorture.
I suppose I could put the declaration in rcutorture.c, but then
0day will tell me to made them static.  :-/

> > > Does it really make a machine boot much faster? Why are people using
> > > synchronous gp primitives if they care about speed? Should we not fix
> > > that instead?
> > 
> > The report I heard was that it provided 10-15% faster boot times.
> 
> That's not insignificant; got more details? I think we should really
> look at why people are using the sync primitives.

I must defer to the people who took the exact measurements.

But yes, once I have that info, I should add it to the commit log.

							Thanx, Paul

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